This invention relates to a multifunctional polymeric additive for hydrocarbon compositions, particularly for lubricating oils. The additive which is a polyampholyte which is a terpolymer of styreneor alkyl styrene/neutralized styrene sulfonate/vinyl pyridine.
The instant exhibits polymers are primarily known for their V.I. (viscosity index) improving properties. Various nonsulfonated polymers have been used as V.I. improvers. Terpolymers of vinyl acetate, an alkyl fumarate and maleic anhydride are taught, for example, in U.S. Pat. No. 3,087,893 and include copolymers made up of from 2 to 15 mole percent of maleic anhydride, 25 to 50 mole percent of an alkyl ester of an alpha, beta-unsaturated dicarboxylic acid, and from 40 to 70 mole percent of an alkylene ester of a C.sub.1 to C.sub.6 monocarboxylic acid. Techniques for forming the polymers are also well-known. For example, a terpolymer of an alkyl fumarate, vinyl acetate, and maleic anhydride can be prepared by the process disclosed in the aforementioned U.S. Pat. No. 3,087,893 or by the improved process described in U.S. Pat. No. 3,136,743.
U.S. Pat. No. 3,637,610 teaches a V.I. improver which is an oil soluble polymer having free carboxylic acid groups which react with amine-containing polymers.
In recent years, there has been a renewed interest in the physical properties of polymeric complexes (i.e., polyampholytes). These materials have a variety of interesting properties since, for all practical purposes, the cations and anions are chemically attached to the molecular structure of the macromolecules. The counterions of any type are not free to move into the bulk solution as found in classical polyelectrolytes. In addition, it is generally assumed that each individual polymer chain possesses an equal number of cations and anions.
Salamone et al., of the University of Lowell (Massachusetts) are investigating ampholytic polymers as a part of their research program. They have studied the solution properties of divinylic cationic-anionic monomer pairs and also cationic-anionic monomer pairs with a neutral comonomer (J. Polym. Sci. Al, 18, 2983 [1980]) which can be incorporated into the ampholytic macromolecular structure through both solution or emulsion polymerization schemes. However, in all of Salamone's work, detailed descriptions of his synthesis is reported. In all instances, the polymerization of the anionic-cationic monomeric species occurred via "ion-pair comonomers that have no nonpolymerizable counterions present" (J. Polym. Sci.-Letters, 15, 487 (1977)). Apparently, the physical and chemical properties of these ion-pair comonomers are different than the individual ions (J. Polym. Sci.-Letters, 15, 487 (1977)).
Polymeric materials are generally considered useful as viscosification agents when dissolved in an appropriate solvent system. The major reason for this viscosity enhancement is due to the very large dimensions of the individual polymer chain as compared to the dimension of the single solvent molecules. Any increase in the size of the polymer chain will produce a corresponding enhancement in the viscosity of the solution. This effect is maximized when the polymer is dissolved in a "good" solvent. Therefore, in general, a hydrocarbon soluble polymer is useful for thickening hydrocarbon solvents, while a water soluble polymer is appropriate for increasing the viscosity of aqueous systems. With regard to nonaqueous solutions, hydrocarbon based solvent soluble nonionic polymers and low charge density sulfonated ionomers are quite useful in this regard and are commonly used materials. However, the solution properties of the former family of materials are controlled primarily through modification of the molecular weight of the polymer and through changes in the level of dissolved polymer. These materials become especially effective at concentrations where the individual polymer chains begin to overlap. This "transition" is commonly referred to in the literature as the chain overlap concentration or simply C*. It should be noted that in most nonionic polymers of commercial interest, a relatively large amount of polymer is required prior to reaching C*. Therefore, this approach is undesirable from an economic viewpoint. Moreover, the rheological properties of many of these nonionic systems have been published. The results of these studies show that, in general, these solutions are shear thinning over all shear rates investigated.
With regard to lightly sulfonated ionomers, the viscosification efficiency of these materials are primarily controlled through formation of an ionically-linked network structure. As long as this network structure remains intact, the sulfonated ionomers possess outstanding viscosity characteristics such as improved thickening efficiency, especially as compared to its nonionic counterpart, the shear thickening. However, these ionic interactions can be dramatically weakened and even completely eliminated if a polar cosolvent, such as an alcohol or an amine, is dissolved into the solution system. However, it should be noted that a polar cosolvent is required in a number of these materials for solvation to occur. Typically, insolubility in xylene (i.e., inability to form a homogeneous single phase solution) occurs in a low charge density sulfonate ionomer solutions if the sulfonation level is greater than approximately 1.0 mole percent. A direct consequence of the addition of these polar cosolvents is a corresponding reduction in solution properties such as thickening efficiency. For example, shear thickening is completely eliminated or sharply reduced in magnitude at relatively low levels of methanol.
This invention teaches that low charge density sulfonate-containing polyampholytes (example: styrene-styrene sulfonate-4 vinyl-pyridine terpolymers) are readily soluble in a single component nonaqueous solvents such as styrene. A polar cosolvent is not required for solvation to take place. Due to this solubility characteristic, these materials are useful in viscosifying nonaqueous solutions. Interestingly, these polyampholytes can possess the unusual property of constant viscosity, i.e., isoviscosity, as the temperature of the solution is varied. It is believed that this temperature invariance is due to the rather strong interionic and intraionic interactions whose strength remains relatively insensitive to temperature changes in the absence of polar cosolvents. These observations are unexpected, since a polar cosolvent is generally required for effective dissolution of low charge density sulfonate ionomers.
The rapid decrease in viscosity of liquids with increasing temperature is well-known. Ideally, for many applications it would be desirable to solve this problem so that viscosity would be insensitive to temperature. Alternatively, it might be desirable to provide liquid systems whose viscosities actually increase with temperature. It is true that with selected polymeric additives it has been possible to reduce substantially the viscosity change with temperature which does occur with most oils and similar systems. These polymer additives, known as viscosity index improvers (or V.I. Improvers) are generally high molecular weight polymers.
The way in which these additives function can be summarized very briefly. In effect, they perform two functions, i.e., thickening, which merely increases fluid viscosity; and Viscosity Index (V.I.) improvement, which corresponds to limited thickening at ambient temperatures and a correspondingly greater thickening at elevated temperatures. This can be accomplished by utilizing a polymeric additive which is poorly solvated by the liquid at ambient temperatures; however, at elevated temperatures the polymer is more highly solvated such that the polymer expands and is a relatively more effective thickener.
While these V.I. Improvers have proven successful commercially, it is important to note that their effect at reducing viscosity changes with temperatures is rather mild. For a typical base oil containing a suitable V.I. Improver, the kinematic viscosity will still decrease by a factor of from 5 to 10 as the temperature increases from 30.degree. to 100.degree. C. Obviously, if it is desired to hold the viscosity roughly constant with such temperature changes, current technology has not offered an appropriate additive system.
U.S. Pat. No. 3,396,136 describes how copolymers of alkenyl aromatic sulfonic acids, when properly neutralized, can be employed as thickeners for nonpolar solvents. Those metal sulfonate systems have been shown to be very effective; however, when employed as two component systems (i.e., ionic polymer plus nonpolar solvent), the variation of viscosity with increased temperature is very conventional and predictable. That is, the solution viscosity decreases markedly as temperature is increased.
U.S. Pat. No. 3,396,136 further teaches "in situ" neutralization of the sulfonic acid polymer which, under some conditions, can result in the availability of a small amount of polar cosolvent--i.e., a solvent for the sulfonate groups about equal in amount to the amount of sulfonate groups which are present. This amount of polar cosolvent is not within the limits of the instant invention, which only optionally requires amounts of the third component (which interacts with the ionomeric groups of the ionomer copolymer) at levels which range from 10 to 600 times the molar equivalence of ionic groups. This level of cosolvent is about one to two orders of magnitude or more higher than employed in the cited art. In addition, the cited patent is restricted to aromatic sulfonate polymers. The instant invention describes other polymers such as sulfonated ethylene propylene terpolymers, sulfonated Butyl, etc., which are a portion of the polymer complex.
U.S. Pat. No. 3,366,430 teaches the gelling of organic liquids by the interaction of polar "associative bonds" which includes hydrogen bonding and "ionic cross-linking". Again, this patent specifies that two components are necessary--the associating polymer (or polymers in some cases) and the nonpolar organic liquid. There is no mention of a third polar cosolvent except to point out that such polar liquids should not be present. Specifically, this patent states (Column 2, line 7) that the hydrocarbon liquids to which this invention is to be applied should not contain a substantial portion of a miscible protolytic liquid such as methanol. It is clear that the language of this patent limits this invention to gels and further, that any amount of polar liquids which are present to an extent where they disrupt those gels are undesirable. The instant invention is distinct from that cited in that amounts of such polar compounds, as will break up gel at ambient conditions, are required and in fact the most preferred state is free of any said gel at ambient temperatures.
U.S. Pat. No. 3,679,382 teaches the thickening of aliphatic hydrocarbons with synthetic organic polymers which contain olefinically unsaturated copolymerizable acids, amides, hydroxyacrylic esters, sulfonic acids, etc. It is emphasized in this patent (Column 3, line 72) that it is critical that in the preparation of such polymers no surface active agent, catalyst or other additive be employed which introduces a metallic ion into the system. Therefore, it is preferred to employ ammonium or amine salts. It is clear that this invention (U.S. Pat. No. 3,679,382) specifically precludes the use of metallic counterions--and is directed towards amine or ammonium derivatives. Finally, this cited patent does describe (Column 7, lines 13-19) that the addition of alcohols will reduce the viscosity of the thickened hydrocarbon and alter flow characteristics thereof.
U.S. Pat. Nos. 3,931,021 and 4,118,361 describe the use of ionic polymers and required cosolvents in an organic liquid and V.I. Improvers. The instant invention represents an improvement over U.S. Pat. Nos. 3,931,021 and 4,118,361. Since cosolvents are not required for the instant invention for many of the polymer compositions, there is more latitude in employing these compositions in controlling viscosity of organic liquids.
U.S. Pat. No. 547,908, now abandoned, teaches a method of controlling the viscosity of the organic liquids with polymeric complexes of a vinyl pyridine copolymer and a sulfonated polymer.